When you cut yourself, new skin grows over the wound because skin cells are 'programmed' to produce new skin. This is vital while you are healing or growing and even means we can grow some cells - such as human skin - in the laboratory.

In contrast to this, a newly fertilised egg contains 'stem' cells, which are capable of becoming any of the hundreds of different types of cell in the body - skin, muscle, brain cells etc.

The challenge faced by Dolly's creators was to take a fully programmed adult cell and return it to this state - de-program it.